Article having turbulation and method of providing turbulation on an article

ABSTRACT

An article includes turbulation material bonded to a surface of a substrate via a bonding agent, such as a braze alloy. In an embodiment, the turbulation material includes a particulate phase of discrete metal alloy particles having an average particle size within a range of about 125 microns to about 4000 microns. Other embodiments include methods for applying turbulation and articles for forming turbulation.

This is a division of application Ser. No. 09/304,276, filed May 3,1999, now U.S. Pat. No. 6,468,669 which is hereby incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to articles that require surface protuberances,such as metal components used in turbine engines. In some embodiments,the invention is more specifically directed to improved techniques forincreasing the heat transfer characteristics on various surfaces of thearticles.

Various techniques have been devised to maintain the temperature ofturbine engine components below critical levels. As an example, coolantair from the engine compressor is often directed through the component,along one or more component surfaces. Such flow is understood in the artas “backside air flow,” where coolant air is directed at a surface of anengine component that is not directly exposed to high temperature gasesfrom combustion. In combination with backside air flow, “turbulators”have been used to enhance heat transfer. Turbulators are protuberancesor “bumps” on selected sections of the surface of the component, whichfunction to increase the heat transfer with the use of a coolant mediumthat is passed along the surface.

Turbulators are generally formed by casting. However, casting cannotreadily be used to apply turbulation to certain areas of a component.For example, it is very difficult to cast protuberances on some portionsof the turbine engine parts, such as on certain sections of internalcavities; in locations where there is restricted molten metal flow; orin areas where mold sections are separated during fabrication. It mayalso be difficult to provide turbulation to some of the externalsurfaces of turbine parts, such as the outer platforms of an enginenozzle.

In some instances, the turbulation on the surfaces of engine componentshas to be repaired or modified while the engine is in service. In otherinstances, it may be necessary to add turbulation to engine componentsduring service or repair, to improve the heat transfer and coolingeffectiveness at specific locations within the component. The additionand repair of turbulation cannot be achieved by the casting process.

One known technique of applying turbulation to an already formedcomponent, is to wire-spray turbulation onto a surface of the substrate.A deficiency associated with such type of turbulation is oxidation ofthe coating, which reduces heat transfer effectiveness. In cased ofsevere oxidation, coating spallation may result with subsequent completeloss of heat transfer benefits.

Further methods for applying turbulation to various types of metalsubstrates would be welcome in the art. There is a need for methods thatare capable of providing turbulation on surfaces that lie withincavities, and on any other surface that is not easily accessible. Thereis a need for methods that are capable of applying protuberances ofdifferent sizes and shapes, and in patterns. In addition, there is aneed for articles having turbulation provided thereon having desirableheat transfer characteristics and durability.

SUMMARY OF THE INVENTION

One embodiment of the present invention calls for a method of providingturbulation on a surface of a substrate, including the steps of applyinga layer on a surface of the substrate, the layer comprising braze alloyand turbulation material; and fusing the layer on the surface of thesubstrate, whereby the braze alloy bonds the turbulation material to thesuperalloy substrate. The layer of material may be applied to thesubstrate in various forms, including a brazing sheet and a slurry. Inaddition, the substrate may be a metal substrate, such as a superalloysubstrate. Another embodiment of the invention calls for an articleincluding a substrate and turbulation material bonded to a surface ofthe superalloy substrate by braze alloy. Still another embodiment callsfor an article including a substrate to which is bonded turbulationmaterial having a particular particle size. Other embodiments include abrazing sheet and a slurry including turbulation material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a partial cross-section of a brazing sheetcontaining protuberances;

FIG. 2 is an illustration of an elevated perspective view of a brazingsheet containing protuberances; and

FIG. 3 is an illustration of a cross section of the brazing sheet ofFIG. 2, as applied to a substrate.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be used with any metallic material or alloy,but is usually used with heat-resistant alloys designed forhigh-temperature environments, such as above 1000° C. As defined herein,“metal-based” refers to substrates that are primarily formed of metal ormetal alloys. Some heat-resistant alloys are “superalloys” includingcobalt-based, nickel-based, and iron-based alloys. In one embodiment,the superalloy is nickel or cobalt based, wherein nickel or cobalt isthe single greatest element by weight. Illustrative nickel-based includeat least about 40 wt % Ni, and at least one component from the groupconsisting of cobalt, chromium, aluminum, tungsten, molybdenum,titanium, and iron. Examples of nickel-based superalloys are designatedby the trade names Inconel®, Nimonic®, Rene® (e.g., Rene®80-, Rene®95alloys, Rene® 142 and Rene® N5), and Udimet®, and include directionallysolidified and single crystal superalloys. Illustrative cobalt-basedinclude at least about 30 wt % Co, and at least one component from thegroup consisting of nickel, chromium, aluminum, tungsten, molybdenum,titanium, and iron. Examples of cobalt-based superalloys are designatedby the trade names Haynes®, Nozzaloy®, Stellite® and Ultimet®.

While the type of substrate can vary widely, it is often in the form ofa turbine engine part formed of a superalloy, such as a combustor liner,combustor dome, bucket or blade, nozzie or vane. Other substrates areturbine parts that are not in the high-pressure stage of the turbineengine, such as in shroud clearance control areas, including flanges,casings, and rings. Such parts may not be formed of a superalloy in viewof lower temperature environments to which such components are exposed.Typical alloys for such components include Inconel® 718, Inconel® 900series, and Waspaloy®.

According to embodiments of the present invention, a layer of materialcontaining at least a braze alloy component and a turbulation materialis utilized to provide turbulation on a surface of a substrate,particularly on a superalloy substrate. As used herein, the term “layer”of material is used to denote a single layer or several discretesub-layers that are sandwiched together. A “layer” of material may haveseveral phases, including a matrix phase having a discrete phasedispersed therein, and several phases defined by sub-layers. The layerof material may be in the form of a free-standing sheet, such as in thecase of a brazing sheet, as well as a slurry containing at least theturbulation material and the braze alloy component. As used herein,“turbulation material” is a material that, upon fusing to a substrate,forms a plurality of protuberances that extend beyond the surface of thesubstrate. These plurality of protuberances together define“turbulation,” which appears as a roughened surface that is effective toincrease heat transfer through the treated substrate. According toseveral embodiments of the present invention, the turbulation materialcomprises a particulate phase comprised of discrete particles bonded tothe substrate. The particulate phase of discrete particles may be formedfrom a coarse powder, described in more detail below with respect toembodiments herein.

In one embodiment of the invention, the layer of material is a brazingsheet, particularly a green braze tape. Such tapes are commerciallyavailable. In an embodiment, the green braze tape is formed from aslurry of metal powder and binder in a liquid medium such as water or anorganic liquid. The liquid medium may function as a solvent for thebinder. The metal powder is often referred to as the “braze alloy”.

The composition of the braze alloy is preferably similar to that of thesubstrate. For example, if the substrate is a nickel-based superalloy,the braze alloy can contain a similar nickel-based superalloycomposition. In the alternative, nickel-based braze alloys orcobalt-based braze alloys are usually used with cobalt-basedsuperalloys. Nickel- or cobalt-based compositions generally denotecompositions wherein nickel or cobalt is the single greatest element inthe composition. The braze alloy composition may also contain silicon,boron, phosphorous or combinations thereof, which serve as melting pointsuppressants. It is noted that other types of braze alloys can be used,such as precious metal compositions containing silver, gold, orpalladium, mixtures thereof, in combination with other metals, such ascopper, manganese, nickel, chrome, silicon, and boron. Mixtures thatinclude at least one of the braze alloy elements are also possible.Exemplary braze alloys include by weight percent, 2.9 boron, 92.6nickel, 4.5 tin; 3.0 boron, 7.0 chromium, 3.0 iron, 83.0 nickel, and 4.0silicon; 19.0 chromium, 71.0 nickel, and 10.0 silicon; 1.8 boron, 94.7nickel, and 3.5 silicon.

A variety of materials are generally used as binders in the slurry forforming the green braze tape. Non-limiting examples include water-basedorganic materials, such as polyethylene oxide and various acrylics.Solvent-based binders can also be used. Additional organic solvent(e.g., acetone, toluene, or various xylenes) or water may be added tothe slurry to adjust viscosity.

The slurry is usually tape cast onto a removable support shoat, such asa plastic sheet formed of a material such as Mylar®. A doctor-bladeapparatus can be used for tape-casting. Substantially all of thevolatile material in the slurry is then allowed to evaporate. Theresulting braze alloy tape usually has a thickness in the range of about1 micron to about 250 microns, and preferably, in the range of about 25microns to about 125 microns.

Braze tapes containing the above-mentioned braze alloy and binder arecommercially available. An example of a commercial product is the Amdryline of braze tapes, available from Sulzer Metco. An exemplary grade isAmdry®100.

The turbulation material that is applied to the green braze tape istypically a coarse powder, being formed of particles having a sizesufficient to form protuberances that function to increase heat transferof the treated component. In many embodiments, the size of the particlesis determined in large part by the desired degree of surface roughnessand surface area (and consequently, heat transfer) that will be providedby the protuberances. Surface roughness is characterized herein by thecenterline average roughness value “Ra”, as well as the averagepeak-to-valley distance “Rz” in a designated area as measured by opticalprofilometry. According to an embodiment, Ra is greater than about 0.1mils, such as greater than about 1.0 mils, and preferably greater thanabout 2.0 mils. Ra is typically less than about 25 mils, more typicallyless than about 10 mils. Similarly, according to an embodiment, Rz isgreater than about 1 mil, such as greater than about 5 mils. Rz istypically less than about 100 mils, more typically less than about 50mils. As used herein, the term “particles” may include fibers, whichhave a high aspect ratio, such as greater than 1:1. In one embodiment,the average size of the turbulation powder particles is in the range ofabout 125 to about 4000 microns, such as about 150 to about 2050microns. In a preferred embodiment, the average size of the powderparticles is in the range of about 180 microns to about 600 microns.

The turbulation material is often formed of a material similar to thatof the substrate metal, which is in turn similar to that of the brazealloy. The turbulation powder, however, may have a higher melting pointor softening point than that of the braze alloy such that theturbulation powder remains largely intact through the fusing operation.Usually, the turbulation powder comprises at least one element selectedfrom the group consisting of nickel, cobalt, aluminum, chromium,silicon, iron, and copper. The powder can be formed of a superalloy bondcoat composition for thermal barrier coating (TBC) systems, such as asuperalloy composition of the formula MCrAIY, where “M” can be variousmetals or combinations of metals, such as Fe, Ni, or Co. The MCrAIYmaterials generally have a composition range of about 17.0-23.0%chromium; about 4.5-12.5% aluminum; and about 0.1-1.2% yttrium; with Mconstituting the balance.

However, it should be emphasized that an important advantage of thepresent process relates to the ability to change the surface “chemistry”of selected portions of the substrate by changing the composition of theturbulation material. For example, the use of oxidation-resistant orcorrosion-resistant metal alloys for the turbulation material willresult in a turbulated surface that exhibits those desirable properties.As another illustration, the thermal conductivity of the turbulationmaterial, which affects the heat transfer, can be increased by using amaterial with a high thermal conductivity, such as nickel aluminidewhich has a thermal conductivity on the order of 228 Btu·in/ft²·hF. Inone embodiment, the turbulation powder is formed of a material having athermal conductivity greater than about 60 Btu·in/ft²·hF, preferablygreater than about 80 Btu·in/ft²·hF, such as greater than about 130Btu·in/ft²·hF. In contrast, prior art casting techniques for producingturbulation usually employ only the base metal material for theprotuberances, thereby limiting flexibility in selecting thecharacteristics of the turbulated surface.

The powder can be randomly applied by a variety of techniques, such assprinkling, pouring, blowing, roll-depositing, and the like. The choiceof deposition technique will depend in part on the desired arrangementof powder particles, to provide the desired pattern of protuberances. Asan example, metered portions of the powder might be sprinkled onto thetape surface through a sieve in those instances where the desiredpattern-density of the protuberances is relatively low.

Usually, an adhesive is applied to the surface of the green braze tapeprior to the application of the turbulation powder thereon. Any brazeadhesive can be used, so long as it is capable of completelyvolatilizing during the subsequent fusing step. Illustrative examples ofadhesives include polyethylene oxide and acrylic materials. Commercialexamples of braze adhesives include “4B Braze Binder”, available fromCotronics Corporation. The adhesive can be applied by varioustechniques. For example, liquid-like adhesives can be sprayed or coatedonto the surface. A thin mat or film with double-sided adhesion couldalternatively be used, such as 3M Company's 467 Adhesive Tape.

In one embodiment, prior to being brazed, the powder particles areshifted on the tape surface to provide the desired alignment that wouldbe most suitable for heat transfer. For example, acicular particles,including fibers, having an elongated shape may be physically aligned sothat their longest dimension extends substantially perpendicular to thesurface of the brazing sheet contacting the substrate. The alignment ofthe powder may be carried out by various other techniques as well. Forexample, a magnetic or electrostatic source may be used to achieve thedesired orientation. In yet another embodiment, individual particles orclusters of particles are coated with braze alloy, and such coatedparticles are placed on an adhesive sheet for application to asubstrate. The adhesive sheet can be formed of any suitable adhesive,provided that it is substantially completely burned-out during thefusing operation. Suitable adhesives are discussed above.

In some embodiments, the turbulation powder is patterned on the surfaceof the braze sheet. Various techniques exist for patterning. In oneembodiment, the powder is be applied to the substrate surface through ascreen, by a screen printing technique. The screen would have aperturesof a pre-selected size and arrangement, depending on the desired shapeand size of the protuberances. Alternatively, the braze adhesive isapplied through the screen and onto the sheet. Removal of the screenresults in a patterned adhesive layer. When the powder is applied to thesheet, it will adhere to the areas that contain the adhesive. By use ofa screen, a pattern may be defined having a plurality of “clusters” ofparticles, wherein the clusters are generally spaced apart from eachother by a pitch corresponding to the spacing of the openings in thescreen. The excess powder can easily be removed, leaving the desiredpattern of particles. As another alternative, a “cookie cutter”technique may be employed, wherein the braze tape is first cut to definea desired turbulation pattern, followed by removal of the excess brazetape. The turbulation powder can then be applied to the patterned tape.In yet another embodiment, particles of the turbulation material arecoated with braze alloy, and the coated particles are adhered onto anadhesive sheet that volatilizes during the fusing step. Here, theadhesive sheet provides a simple means for attachment of the turbulationmaterial to the substrate prior to fusing, but generally plays no rolein the final, fused article.

In another embodiment, the turbulation powder is mixed with the othercomponents of the green braze tape, such as braze alloy powder, binderand solvent, during formation of the green braze tape, rather thanproviding the turbulation powder on a surface of the already formedtape. The turbulation powder in turn forms a dispersed particulate phasewithin the green braze tape.

The removable support sheet, such as Mylar® backing is then detachedfrom the green braze tape. The tape is then attached to a portion of thecomponent-substrate where turbulation is desired. As an example, anadhesive may be employed. Any adhesive suitable for attaching the tapeto the substrate material would be suitable, provided that it completelyvolatilizes during the fusing step.

Another simple means of attachment is used in some embodiments. Thegreen braze tape can be placed on a selected portion of the substrate,and then contacted with a solvent that partially dissolves andplasticizes the binder, causing the tape to conform and adhere to thesubstrate surface. As an example, toluene, acetone or another organicsolvent could be sprayed or brushed onto the braze tape after the tapeis placed on the substrate.

Following application of the green braze tape to the substrate, theturbulation material is fused to the substrate. The fusing step can becarried out by various techniques, such as brazing and welding.Generally, fusing is carried out by brazing, which includes any methodof joining metals that involves the use of a filler metal or alloy.Thus, it should also be clear that braze tapes and braze foils can beused in fusing processes other than “brazing”. Brazing temperaturesdepend in part on the type of braze alloy used, and are typically in therange of about 525° C., to about 1650° C. In the case of nickel-basedbraze alloys, braze temperatures are usually in the range of about 800°C. to about 1260° C.

When possible, brazing is often carried out in a vacuum furnace. Theamount of vacuum will depend in part on the composition of the brazealloy. Usually, the vacuum will be in the range of about 10⁻¹ torr toabout 10⁻⁸ torr, achieved by evacuating ambient air from a vacuumchamber to the desired level.

In the case of turbulation being applied to an area which does not lenditself to the use of a furnace, such as when the component itself is toolarge to be inserted into a furnace, a torch or other localized heatingmeans can be used. For example, a torch with an argon cover shield orflux could be directed at the brazing surface. Specific, illustrativetypes of heating techniques for this purpose include the use of gaswelding torches (e.g., oxy-acetylene, oxy-hydrogen, air-acetylene,air-hydrogen); RF (radio frequency) welding; TIG (tungsten inert-gas)welding; electron-beam welding; resistance welding; and the use of IR(infra-red) lamps.

The fusing step fuses the brazing sheet to the substrate. When the brazematerial cools, it forms a metallurgical bond at the surface of thesubstrate, with the turbulation material mechanically retained withinthe solidified braze matrix material.

In another embodiment of the invention, the layer of material is abrazing sheet in the form of a metal foil having first and secondsurfaces. The foil is formed of a metallic material similar to that ofthe substrate, such as a braze alloy like that described for theprevious embodiment. Thus, if the substrate is a nickel-basedsuperalloy, the foil material may be a nickel-based superalloy. Thebraze alloy composition for foils, which does not contain a binder as inthe case of braze tapes, may contain silicon, boron, or combinationsthereof, which serve as melting point suppressants. Other braze alloycompositions may also be suitable, such as those comprising cobalt oriron; or the precious metal compositions described previously. The foilusually has a thickness of about 0.1 micron to about 2500 microns, andpreferably, about 25 microns to about 200 microns.

Various techniques can be used to make such a foil. In the firsttechnique, a mixture of metallic powder material and binder is tapecastonto a removable support sheet. The support sheet is removed, and theremaining green sheet is then sintered into a “pre-form” foil, e.g., byusing a vacuum heat treatment. The sintering temperature is dependent onvarious factors, such as the composition of the foil-alloy, the size ofthe powder particles, and the desired density of the foil. This processis typically called a “tape-cast pre-form” technique.

According to a second alternative technique, a metallic powder materialis deposited onto a support sheet as a thin layer of metal. Variousthermal spray techniques are usually used for the deposition, such asvacuum plasma deposition, HVOF (high velocity oxy-fuel), or air plasma(AP) spray. Other deposition techniques could be employed as well, e.g.,sputtering or physical vapor deposition (PVD). The support sheet is thenremoved, leaving the desired metal foil.

A third technique for making the foil is sometimes referred to as anamorphous metal ribbon technique. In this process, the metallic powdermaterial is melted, and the molten material is poured onto a high-speedroller that very rapidly quenches the molten material. The quenchedmaterial is ejected from the roller as a ribbon. Braze foils arecommercially available from various sources, such as Wesgo and AlliedSignal Company. In general, the braze foil differs from the green brazetape described above in that the foil is in a sintered, densified formbefore application of the turbulation powder and subsequent fusing to asubstrate.

The turbulation powder is applied to the first surface of the brazefoil. The powder generally has the same characteristics as the powderdescribed for the previously described embodiment incorporating a greenbraze tape. As in that case, the powder is usually formed of a materialsimilar to that of the substrate metal, which in turn is similar to thatof the braze alloy. Thus, the powder is usually nickel-based, and mayhave a composition of the formula MCrAIY, as described previously. Thetechniques described above can be used to apply the powder, such asthermal spray or casting.

Usually, an adhesive is applied to the second surface of the foil, priorto the application of the turbulation powder. The adhesive can beselected from those described previously, provided that it adheres tothe metallic foil and it completely volatilizes during the subsequentfusing step. Illustrative adhesives are those that were describedpreviously, e.g., polyethylene oxide and various acrylics. Techniquesfor applying the adhesive would also be similar or identical to thosedescribed previously.

Moreover, the powder particles can be shifted and aligned as describedabove, based on the required heat transfer characteristics for thesubstrate surface. Similarly, the powder particles can also be patternedon the surface of the foil by various techniques.

In some instances, the substrate surface to which the foil will beattached is curved. In such a case, it may be desirable to provide thefoil with an identical curvature. Relatively thin foils may be easilybent to match the curvature of a substrate. Foils of greater thicknessusually are not flexible, but can be shaped by other techniques. As anexample, a removable support sheet may be employed during fabrication,which sheet has the desired curvature of the substrate. The brazematerial can be deposited on the support sheet by the techniquesdescribed previously, e.g., thermal spraying or casting (for example,liquid metallic casting without a binder, or powder-slurry casting, witha binder). The turbulation powder can then be deposited on the foil, asalso described previously. The turbulation-containing foil which has thedesired curvature can then be detached from the support sheet.(Alternatively, the turbulation powder could be applied to the foilsurface after the support sheet is removed).

The turbulated braze foil is cut to a size appropriate for the site onthe substrate where turbulation is to be formed. The foil can then beattached to that portion of the substrate. As an example, the firstsurface of the foil, i.e., the surface opposite that which is coatedwith the turbulation powder, could be attached to the substrate with anadhesive sheet or adhesive composition. Any adhesive suitable forattaching the foil to the substrate metal should be suitable, as long asit completely volatilizes during the fusing step. Illustrative adhesivesare those that were described previously.

Alternativiely, the braze foil could be attached by mechanical means. Insome preferred embodiments, the foil is locally welded to the substratesurface at a few locations (spot welding). A variety of heatingtechniques could be employed, such as TIG (tungsten inert-gas) welding,resistance welding, gas welding (e.g., with a torch); RF welding,electron-beam welding; and IR lamp methods.

Fusing of the foil to the substrate can then be undertaken as describedpreviously, with brazing often being used for this step. Brazingtemperatures will again depend in part on the type of braze alloy usedfor the foil, and are typically in the range of about 525° C. to about1650° C. In the case of nickel-based braze alloys as described above,braze temperatures are usually in the range of about 800° C. to about1260° C. The fusing step fuses the foil to the substrate, as describedpreviously, and may be carried out in a vacuum furnace. Alternatively,brazing may be accomplished through use of a torch or other heatingtechnique (e.g., the welding techniques mentioned above) can be used forfusing the foil to the substrate, as an alternative to the vacuumfurnace.

According to another embodiment, the substrate is coated with a layer ofmaterial in slurry form. That is, in contrast to the embodimentsdescribed above, a brazing sheet (in the form of a green braze tape orbrazing foil) is not used. Rather, a slurry containing a liquid medium,braze alloy powder, and turbulation powder is directly applied to asurface of the substrate. The slurry is dried, and then the coatedsubstrate is heated such that the braze softens to form a film thatbonds the turbulation powder to the substrate. The slurry may optionallycontain a binder, and the liquid medium may function as a solvent forthe binder. Use of a binder is desirable in cases where handling of thecomponent is necessary after drying of the slurry and before fusing,such as transporting the coated component to a furnace.

The liquid medium may be water, an organic component such as acetone,toluene, or various xylenes, or mixtures of water and an organiccomponent. The turbulation powder, braze alloy powder, and binder may beformed of materials described above. For example, the turbulation powdergenerally includes at least one element selected from the groupconsisting of nickel, cobalt, iron, and copper. In one embodiment, theturbulation powder has a composition according to the formula MCrAIY,where “M” is a metal from the group Fe, Ni, or Co, or combinationsthereof. MCrAIY materials may have a composition range of about17.0-23.0% Cr, 4.5-12.5% Al, and about 0.1-1.2% Y, the balance M. In oneembodiment, M is Ni. By way of example, binders include water-basedorganic materials (or combinations of materials), such as polyethyleneoxide and various acrylics. Solvent-based binders can also be used.

The slurry itself generally contains turbulation powder, braze alloy,and binder. The amount of braze alloy is chosen relative to theturbulation powder in an amount sufficient to bond the particles of theturbulation powder to the substrate, such as about 1 to 40 wt % brazealloy and the balance (about 60 to 99 wt %) turbulation powder. Theamount of binder is generally present in an amount to ensure sufficientgreen strength for handling while minimizing the volume of binderburnout, such as about 1 to 20 wt % of the slurry.

In the embodiments described above, the structure of the componentafter-fusing includes a solidified braze film that forms a portion ofthe outer surface of the component, and protuberances that extend beyondthat surface. The protuberances are generally made up of a particulatephase comprised of discrete particles. The particles may be arranged ina monolayer, which generally has little or no stacking of particles, oralternatively, clusters of particles in which some particles may bestacked on each other. Thus, after fusing, the treated component has anouter surface defined by the film of braze alloy, which has aparticulate phase embedded therein. The film of braze alloy may form acontinuous matrix phase. As used herein, “continuous” matrix phasedenotes an uninterrupted film along the treated region of the substrate,between particles or clusters of particles. Alternatively, the film ofbraze alloy may not be continuous, but rather, be only locally presentto bond individual particles to the substrate. In this case, the film ofbraze alloy is present in the form of localized fillets, surroundingdiscrete particles or clusters of particles. In either case, thinportions of the film may extend so as to coat or partially coatparticles of the turbulation powder.

FIG. 1 illustrates an embodiment of the present invention after fusing.As shown, component 20 has an internal cavity 22 through which“backside” coolant fluid is passed. The internal surfaces of thecomponent 22 are treated according any one of the techniques heretoforedescribed, to form a braze alloy film 24 that forms a continuous matrixphase, and a discrete particulate phase 26 comprised of turbulationmaterial. In FIG. 1, the particles of particulate phase are randomlyarranged, but may alternatively be arranged according to a predeterminedpatterned as described above. While the component 20 is shown in partialcross section to have a cylindrical form, it may take on any one of themany shapes and sizes of components of state of the art turbine engines,for example. While turbulation is shown on an internal surface ofcomponent 20, it may be provided externally in components where hotgases travel through internal cavities, and cooling fluids pass over theexterior surfaces.

The average height h of the protuberances as measured from the substrateis generally on the order of the average particle size of the particlesof the turbulation material, such as about 125 microns to about 4000microns or about 150 microns to about 2050 microns. The height h mayalso be within a range of about 180 to about 600 microns. The thicknessof the braze alloy film 24 overlying the substrate is generally chosento ensure adequate roughness and ensure an increase in surface area,provided by the particulate phase 26, while also ensuring adequateadhesion of the particles to the substrate. The thickness may be on theorder of about 20 microns to 100 microns, more particularly, 30 to 70microns. In one embodiment, the thickness is approximately 50 microns.It is noted that the braze alloy film 24 mainly forms thin layer shownin FIG. 1, but also may form a thin coating overlying the individualparticles of the particulate phase 26.

In another embodiment of the present invention, the protuberances can beformed on the brazing sheet by the use of a mold, rather than byindividual, discrete particles of turbulation powder. The mold includesrecesses (within one of its major surfaces) suitable for replicating thesize and shape of the protuberances. The mold can be a sheet of rubber(for example, an RTV compound) or any synthetic material. Alternatively,the mold can be formed of a ceramic or metallic material. This type ofmold can itself be made from an existing turbulation-surface bytechniques well-known in the art.

One of the mold techniques for this embodiment may be referred to as“green casting”. According to this technique, the recesses of the moldare filled with a slurry material. The slurry contains a liquid medium,the turbulation powder, braze alloy powder, and optionally a binder. Theliquid medium may function as a solvent for the binder, effective toenhance the mixing of alloy and binder. The liquid medium may includewater, an organic component such as acetone, aromatic solvents such astoluene, isopropanol, or various xylenes, or mixtures of water and theorganic component. The braze alloy and turbulation material compositionscan be as described previously. Suitable binders have also beendescribed for other embodiments, e.g., water-based organic materials orsolvent-based binders.

The recesses can be filled with the slurry by any convenient technique,such as casting or trowelling. Sometimes, a small amount of a releaseagent, such as a stearate or a silicone-based material, is applied toall or part of the surfaces of the recesses before filling with theslurry, to promote separation from the mold at the appropriate time.

The open face of the filled mold is then usually placed against asurface of the brazing sheet, which can be a green braze tape or brazefoil, as described previously. The sheet surface may have an adhesivelayer applied thereto, to enhance adhesion between the moldedprotuberances and the sheet. The mold can then be pulled or cut off thesheet, leaving the exposed protuberances.

Alternatively, the braze tape could be formed “in-situ” on the open faceof the mold, which is filled with turbulation powder and braze alloy. Inother words, a slurry of tape-forming metal powder, binder, andoptionally, solvent, could be applied to the open face of the mold.Evaporation of the volatile material in the slurry results in a tape inthe green state. The slurry may be heated to increase the evaporationrate. The mold can then be removed to expose the protuberances, beforeor after application to the substrate.

As yet another alternative, the braze foil could be formed in-situ onthe open face of a mold, such as a metal or ceramic mold. Thisalternative could be carried out by depositing the turbulation material,in molten form, onto the open face of the mold, by one of the techniquesdescribed previously, e.g., a thermal spray technique or casting (here,usually liquid metal casting). After the recesses in the mold have beenfilled, a thin layer of braze alloy, e.g., up to about 125 microns, canbe applied over the mold—again by casting or thermal spray. (Sometimes,the surface of the filled mold can be ground to a level state beforedeposition of the braze alloy). The protuberances remain affixed to thethin braze alloy sheet after removal of the mold.

With respect to the slurry for filling the recesses in the mold, thebraze alloy is present in an amount that is sufficient to promote liquidphase sintering of the turbulation powder within the braze alloy liquidmatrix, and not so much so as to cause the protrusion to collapse duringsintering within a molten pool of braze alloy. The amount of braze alloyis chosen relative to the turbulation powder, such as about 1 to 40 wt %braze alloy and the balance (about 60 to 99 wt %) turbulation powder.The amount of binder is generally present in an amount to ensuresufficient green strength for handling while minimizing the volume ofbinder burnout, such as about 1 to 20 wt % of the slurry.

In the above examples of this embodiment, the particle size of theturbulation powder need not be as large as described above in connectionwith, since the protuberances may be formed by groups of particles.Generally, the particle size is within a range of about 1 micron toabout 4000 microns, such as about 10 microns to about 2000 microns,preferably not greater than about 500 microns. In one embodiment, theparticle size is within a range of about 25 to about 180 microns.

Regardless of which of these turbulation-mold alternatives is employed,the resulting article is a layer of material in the form of a brazingsheet that includes the desired protuberances situated on a brazematerial, for attachment to the component. The method of attachment willdepend in part on whether the brazing sheet is a braze tape or a brazefoil, with suitable techniques, in each instance, being describedpreviously.

FIG. 2 is an illustration of an elevated perspective view of a brazingsheet containing protuberances according to this embodiment of thepresent invention. As illustrated, brazing sheet 10 includesprotuberances 12 that are provided to be exposed to a coolant medium.While the protuberances are shown to have a generally semi-sphericalshape, they may take on other shapes as well to meet desired roughnessand surface area characteristics, to obtain a desired heat transferenhancement.

FIG. 3 illustrates a partial cross-sectional view of the brazing sheet10 shown in FIG. 2, applied to a substrate 8. As illustrated,protuberances 12 of brazing sheet 10 have a height h on the order ofabout 125 microns to about 4000 microns, such as about 150 microns toabout 2050 microns, similar to the protuberances of the previouslydescribed embodiments. The height may also be within a range of about180 microns to about 600 microns. The protuberances are made up ofclosely packed grains or particles 14 of the turbulation powder. Brazealloy 16 forms a thin film overlying the substrate, and also fills inspaces between the particles 14 of the turbulation powder within theprotuberances. The braze alloy forming an intergranular phase betweenparticles 14 is effective to ensure efficient heat transfer through theprotuberances by reducing porosity.

As mentioned above, the size and pattern of turbulation can be readilyadjusted to provide maximum heat transfer for a given situation.Usually, the protuberances are substantially semi-spherical in shape,either assuming the shape of the mold recesses or the shape of particlesof the turbulation powder, according to the different embodimentsdiscussed herein. Other shapes are possible, such as cones, truncatedcones, pins, or fins. The number of protuberances per square cm ofsubstrate will depend upon various factors, such as their size andshape. In one embodiment, the number of protuberances is sufficient tocover about 40% to about 95% of the particular substrate surface, i.e.,the particular area of the substrate that is treated according toembodiments of the present invention, which ranges from a small interiorcooling channel surface, to the entirety of an exposed surface of aturbine engine component, for example.

The application of turbulation material according to embodiments of thepresent invention is effective to increase surface area of thesubstrate. For example, A/A₀, where A is the surface area of the treatedregion of the component and A₀ is the surface area of the same region ofthe component in untreated form (generally a smooth surface), isgenerally at least about 1.05, typically at least about 1.20. A/A₀ isgenerally less than about 4.0, typically less than about 2.5

While braze alloy have been described above in connection with theforegoing embodiments as a preferable class of bonding agents, otherbonding agents can be used. For example, high temperature epoxies may beused in less demanding environments, such as on non-superalloycomponents subjected to lower operating temperatures.

In most embodiments, the turbulation (i.e., the “roughness” provided bythe protuberances) is present to enhance the heat transfercharacteristics for the underlying component. The enhanced heat transfercharacteristics in turn result in a desirable temperature reduction forspecified regions of the component, leading to a desirable reduction inthermal stress. Moreover, by tailoring the size and spacing of theprotuberances, the heat transfer enhancement can also be adjusted, whichin turn results in a reduction in the thermal and stress gradients forthe component.

Embodiments of the present invention have shown improvement in heattransfer over wire-sprayed turbulation. For example, embodiments of thepresent invention have provided a heat transfer enhancement greater than1.52, such as greater than about 1.60, at a jet Reynolds number of40,000. In contrast, wire-spray techniques have been shown to formturbulation that has a heat transfer enhancement on the order of 1.3 to1.52 at a jet Reynolds number of 40,000. Particular embodiments of thepresent invention have shown a heat transfer enhancement of about 1.70to about 1.82 at a jet Reynolds number of 40,000. The heat transferenhancements are normalized to 1.0, denoting a smooth, untreatedsurface. Heat transfer values were measured by embedding thermocouplesinto the turbulated substrate for temperature measurement. Animpingement plate was mounted above the turbulated substrate, and aselected amount of heat was applied to the opposite side of theturbulated substrate, i.e., the side without the protuberances. Coolantair was then blown through the holes in the impingement plate, onto theturbulated surface. The amount of energy required to keep the turbulatedsurface at a selected temperature was then measured (the higher therequired temperature, the greater the enhancement in heat transfer). Thetreated samples were then compared to smooth, untreated baselinesamples.

According to embodiments of the present invention, by keeping theturbulation close to the surface of the substrate, pressure drop of thecoolant medium flow across the cooled surface is reduced and the fincooling efficiency is improved. For example, in one embodiment, theheight of the turbulation is kept below 600 microns, more particularly,less than about 375 microns. The particle size may be less than about600 microns, more particularly, less than about 375 microns to ensurethat the turbulation material is close to the surface to improve finefficiency.

It is noted that while temperature measurements were taken in connectionwith impingement cooling (coolant air flow perpendicular to the surfaceof the substrate), cooling may be effected by convection (coolant airflow parallel to the surface of the substrate) as well in practical use.

Further, embodiments of the present invention demonstrated a markedimprovement in oxidation resistance over conventional wire-sprayedturbulation. Samples were thermally cycled between room temperature anda 2,000° F., with a 45 min. hold at 2,000° F. The cycling revealedvirtually no oxidation of the turbulated surface at 200 furnace cyclesand only slight oxidation at 400 furnace cycles. In contrast, wire-sprayturbulation was found to demonstrate significant oxidation at 200furnace cycles, leading to premature spalling of turbulation.

As described above, turbulation is usually used in conjunction with acoolant medium that is being directed against or across a component usedin a high temperature environment. It should be understood that whilethe coolant medium is usually air, it could also be composed of otherfluids such as water.

EXAMPLES

The following examples are merely illustrative, and should not beconstrued to be any sort of limitation on the scope of the claimedinvention. All parts are provided in weight percent, unless otherwiseindicated.

Example 1

A commercial braze tape was used in this example: Amdry®100(composition: 10% by weight silicon; 19% by weight chrome, balancenickel). The tape had a thickness of about 25-50 microns, and was coatedwith a very thin organic adhesive. A coarse NiCrAIY bond coat powder wasemployed, having an approximate composition as follows: 68 wt. % Ni, 22wt. % Cr, 9 wt. % Al, and 1 wt. % Y. The powder had an average particlesize (diameter) of 50-80 mesh, i.e., 180-300 microns, and was manuallyapplied to the braze tape surface. The tape was then cut to a size ofabout 5 cm×5 cm, and attached to a portion of a nozzle cavity surface ofa turbine engine component formed from a nickel-based superalloy. Asolvent (acetone) was then sprayed onto the tape, causing it toplasticize, conform, and adhere securely to the cavity surface.

The nozzle cavity was then vacuum-brazed at a brazing temperature ofabout 2150° F. (1177° C.), using a vacuum furnace held at about 10⁻⁵torr. The turbulation powder was completely fused to the cavity surface.The surface exhibited a rough texture because of the presence of theturbulation. The protuberances were substantially semi-spherical inshape. The measured Ra value was about 2.7 mils (68.6 microns), and theRz value was about 13.5 mils (343 microns). At a jet Reynolds number of20,000, a 1.7 heat transfer enhancement was measured, and at a jetReynolds number of 40,000, a 1.9 heat transfer enhancement was measured.

Example 2

The type of NiCrAIY bond coat powder used in Example 1 was used here.The powder was applied to the braze tape surface (coated with anadhesive) through a 40 mesh screen (425 micron particle size maximum),to form a pattern on the surface. The tape was then cut and attached toa portion of a nozzle cavity surface, as in the previous example.Vacuum-brazing was also carried out as in Example 1, completely fusingthe turbulation to the cavity surface, in the desired pattern. The heattransfer measurements for this example, performed as in Example 1,yielded a heat transfer enhancement value of 1.9 at a jet Reynoldsnumber of 40,000.

Example 3

A braze foil having the same alloy as in Example 1, was used in thisexample. The foil was cut to a size of about 5 cm×5 cm. A patternedlayer of adhesive was screen-printed onto the foil surface. The NiCrAIYbond ccat powder used in Example 1 was then manually distributed ontothe adhesive-coated surface. After the excess powder was removed fromthe surface, the foil was spot-welded onto a portion of a nozzle cavitysurface, and vacuum-brazed thereto, as in Example 1 (same brazingconditions). As in Example 2, the patterned turbulation was completelyfused to the cavity surface. At a jet Reynolds number of 20,000, a 1.73heat transfer enhancement was measured, and at a jet Reynolds number of40,000, a 1.9 heat transfer enhancement was measured.

While turbulation was applied to in nozzle cavity, a wide variety ofother components may also be treated. For example, other superalloycomponents including combustor liners, combustor domes, buckets orblades, or shrouds. Non-superalloy components used in lower temperatureapplications may also be treated. For example, shroud clearance controlareas, including flanges, casings, and rings may be advantageouslytreated. In these embodiments, use of turbulation permits more accuratecontrol of the diameter of the flowpath shroud, thereby decreasing theclearance between the blade tip and shroud surface and increasingefficiency. In view of the lower temperature requirements for thematerials of such components, the braze alloy may be replaced withanother bonding agent such as a high temperature epoxy or solder, forexample. The application of the turbulation material may be by brazingsheet or by slurry containing the bonding agent and the turbulationmaterial, as described above.

As described above, the term “turbulation” has been used to denote aroughened surface comprised of a plurality of protuberances that areeffective to increase heat transfer through a treated component. Theroughened surface in some embodiments appears sandpaper-like inappearance. The increase in heat transfer is believed to be largely dueto the increased surface area of the treated component. Turbulation mayalso increase heat transfer by modifying the coolant medium flowcharacteristics, such as from laminar flow to turbulated flow along thesurface, particularly where the turbulation material is principallyformed of large particle size material.

According to embodiments of the present invention, methods are providedthat permit application of turbulation to surfaces that are not easilyaccessible, to provide improved heat transfer. Further, embodiments ofthe present invention enable formation of protuberances of varying sizesand shapes, and in a patterns, if desired. In addition, according toembodiments of the present invention that utilize a layer includingturbulation material and a bonding agent such as braze alloy, improvedresistance to oxidation and corrosion at high temperatures may beachieved, as well as improved heat transfer effectiveness.

Having described preferred embodiments of the present invention,alternative embodiments may become apparent to those skilled in the artwithout departing from the spirit of this invention. Accordingly, it isunderstood that the scope of this invention is to be limited only by theappended claims.

What is claimed:
 1. A method of providing turbulation on a surface of asuperalloy substrate, comprising the steps of: applying a layer on asurface of the superalloy substrate, the layer comprising braze alloyand turbulation material, wherein said turbulation material has anaverage particle size in the range from about 180 microns to about 4000microns, and wherein said layer comprises a free-standing brazing sheet,said brazing sheet selected from the group consisting of a. a greenbraze tape comprising a braze alloy and a binder: and b. a braze metalfoil, wherein said turbulation material is provided on a surface of saidbraze metal foil; and fusing the layer on the surface of the superalloysubstrate, whereby the braze alloy melts and bonds the turbulationmaterial to the superalloy substrate.
 2. The method of claim 1, whereinthe braze alloy forms a continuous matrix phase film.
 3. The method ofclaim 1, wherein the braze alloy forms a discontinuous film.
 4. Themethod of claim 3, wherein the turbulation material comprises particles,and the braze alloy forms fillets that bond the particles to thesubstrate.
 5. The method of claim 1, wherein the superalloy substratecomprises nickel-based or a cobalt-based alloy.
 6. The method of claim5, wherein the superalloy substrate comprises a nickel-based alloy, andincludes at least about 40 wt % nickel and at least one component fromthe group consisting of cobalt, aluminum, chromium, silicon, tungsten,molybdenum, titanium, and iron.
 7. The method of claim 1, wherein theturbulation material comprises at least one element from the groupconsisting of nickel, cobalt, aluminum, chromium, silicon, iron, andcopper.
 8. The method of claim 7, wherein the turbulation material is analloy having a composition MCrAIY, wherein “M” comprises at least onematerial selected from the group consisting of iron, nickel and cobalt.9. The method of claim 1, wherein the braze alloy comprises at least onemetal selected from the group consisting of nickel, cobalt, iron, aprecious metal, and a mixture thereof.
 10. The method of claim 9,wherein the braze alloy comprises at least about 40 wt % nickel.
 11. Themethod of claim 9, wherein the braze alloy further comprises a componentfor lowering the melting point of the braze alloy.
 12. The method ofclaim 11, wherein the component is selected from the group consisting ofsilicon, boron, phosphorous, and combinations thereof.
 13. The method ofclaim 1, wherein the turbulation material forms a plurality ofprotuberances that extend beyond the surface of the superalloy substrateto define a turbulated surface.
 14. The method of claim 13, wherein theturbulated surface has a heat transfer enhancement greater than 1.52 ata jet Reynolds number of 40,000.
 15. The method of claim 14, wherein theheat transfer enhancement is not less than about 1.60.
 16. The method ofclaim 1, wherein the average particle size is within a range of about180 microns to about 2050 microns.
 17. The method of claim 1, whereinthe brazing sheet comprises a green braze tape and includes said brazealloy and a binder.
 18. The method of claim 17, wherein the turbulationmaterial is mixed within the green braze tape prior to fusing on thesubstrate.
 19. The method of claim 17, wherein the turbulation materialis applied to a surface of the green braze tape prior to fusing on thesubstrate.
 20. The method of claim 19, wherein the turbulation materialis patterned on the surface of the green braze tape.
 21. The method ofclaim 17, wherein the green braze tape is formed by depositing a slurryof the braze alloy and binder onto a removable support sheet, and dryingthe slurry.
 22. The method of claim 17, wherein the green braze tape hasa thickness in a range of about 1 micron to about 250 microns.
 23. Themethod of claim 17, wherein the green braze tape is attached to thesurface of the superalloy substrate by an adhesive.
 24. The method ofclaim 17, wherein prior to the step of fusing, the green braze tape isexposed to a solvent that plasticizes the binder, causing the greenbraze tape to conform to the surface of the superalloy substrate. 25.The method of claim 1 wherein the metal foil has a thickness in a rangeof about 0.1 microns to about 250 microns.
 26. The method of claim 1,wherein the turbulation material forms a plurality of protuberances thatextend beyond the surface of the superalloy substrate to define aturbulated surface, the turbulation material comprises a turbulationpowder, and the brazing sheet is formed by (i) providing a mold having asurface in which a plurality of recesses are formed, (ii) filling theplurality of recesses in the mold with a slurry containing saidturbulation powder and braze alloy, to form said protuberances (ii)placing a material component on the mold, in contact with theturbulation material, said material component comprising braze alloy andforming a continuous sheet, (iii) removing the continuous sheet with theprotuberances attached thereto.
 27. The method of claim 26, wherein saidmaterial component comprises a metal foil.
 28. The method of claim 26,wherein said material component comprises a green braze tape.
 29. Themethod of claim 26, where n said material component comprises a slurry.30. The method of claim 26, wherein the turbulation material has anaverage particle size within a range of 1 micron to about 4000 microns.31. The method of claim 1, wherein the step of fusing is carried out bybrazing, at a temperature of about 525° C. to about 1650° C.
 32. Themethod of claim 31, wherein the step of brazing is carried out in avacuum furnace.
 33. The method of claim 1, wherein the step of fusing iscarried out by locally heating a portion of the substrate.
 34. A methodfor providing turbulation on a surface of a metal substrate, comprisingthe steps of: providing a slurry on a surface of a metal substrate, theslurry comprising braze alloy and turbulation material, wherein theturbulation material has an average particle size within a range ofabout 180 microns to about 4000 microns; and fusing the trurbulationmaterial to the substrate by melting said braze material wherein theturbulation material forms protuberances that extend beyond the surfaceof the substrate.
 35. The method of claim 34, wherein the turbulationmaterial comprises at least one element from the group consisting ofnickel, cobalt, iron, and copper.
 36. The method of claim 34, whereinthe turbulation material comprises an alloy having a composition MCrAIY,wherein “M” comprises at least one material selected from the groupconsisting of iron, nickel and cobalt.
 37. The method of claim 34,wherein the substrate comprises a superalloy.